Inorganic pigments are coloured compounds with a high thermal and chemical stability used to colour ceramic bodies. Pigments are used in a wide range of applications including paints, inks, plastics, rubbers, ceramics, enamels, and glasses. Many of these materials consist of an oxide matrix doped with transition metal cations that act as chromophore. One of the main research activities in this field is the search for new inorganic structures that, once doped with proper chromophore ions, result in new pigments that are cheaper, less toxic, or have more attractive shades than the currently used pigments. The brilliant colours of the inorganic pigments are usually due to selective absorption of visible light.
Numerous Co2+ (3d7) based oxides exhibit strong violet or blue coloration as well as a light pink hue and have been used as pigments. The UV-vis-NIR absorption properties are strongly related to the structural features and especially to the local environment of transition metals. Commercially used blue pigments are Co—Cr—Al (P.B.36) and Co—Al (P.B.28) and others are Ultramarine blue, Prussian blue, cobalt phosphates such as Co3(PO4)2, Co2SiO4 (olivine), Co-Willemite, (Zn2SiO4) etc. There is an increasing demand to develop new NIR reflective pigments which are can be used for cool roof applications. A cool roof reflects and emits the sun's heat back to the sky instead of transferring it to the building. Replacing conventional pigments with “cool pigments” that absorb less NIR radiation can provide coatings similar in color to that of conventional roofing materials, but with higher solar reflectance. Taking into account these previous investigations, the aim of the present study is to develop and characterize for the new low-toxicity and NIR reflecting ceramic pigments.
Inorganic pigments comprising of cobalt and aluminum having spinel structure most commonly produced by high temperature calcinations of raw materials such as cobalt (II) oxide(CoO), and aluminum(III) oxide(Al2O3). Inorganic pigments comprising cobalt and aluminum having the spinel structure are used in a variety of applications including paints and polymers. Paints and polymers containing such inorganic pigments are often exposed to UV radiation and other environmental conditions. The intensity and color of paints and polymer products containing such inorganic pigments tend to degrade upon extended exposure to the environment. These conventional cobalt aluminate spinel pigments are synthesized at a high temperature (1300° C.).
U.S. Pat. No. 5,252,126, Oct. 12, 1993, describes a process for the preparation of neutral blue vanadium-zirconium inorganic blue pigment. In order to produce such pigments, a zirconium free mixture of powdery ZrO2 and SiO2 source, a vanadium compound and a phosphorus compound with an atomic ratio of Zr:Si:V:P=(0.95-1.10):(0.05-0.20):(0.005-0.03) as well as of fluoride mineraliser is annealed after intensive grinding at 700° C. to 900° C. The neutral blue pigment with a rather high color intensity and good reproducibility are doped in the host lattice with phosphorus addition to vanadium. However these pigments have less colour intensity than commercially available CoAl2O4 spinel.
A novel non-toxic intense blue near-infrared reflecting inorganic pigments having the general formula Sr1-xLaxCu1-yLiySi4O10 (x=y ranges from 0 to 0.5) were developed as viable alternatives to existing blue colorants. (Sheethu Jose, M. L. Reddy Dyes and Pigments 98 (2013) 540-546). The substitution of La3+ for Sr2+ and Li+ for Cu2+ in SrCuSi4O10 gently changes the color of the pigment from sky-blue to intense blue. The developed pigments exhibit intense blue color with impressive NIR solar reflectance (67%) and thermally stable.
U.S. Pat. No. 3,748,165, Jul. 24, 1973 describes a process for the preparation of an improved inorganic pigment of spinel structure which comprises of about 15 to about 50 mole percent of nickel aluminate in cobalt aluminate. The improved pigment retains a greater degree of blueness when diluted 1 to 10 with TiO2 pigment than either CoAl2O4 or NiAl2O4 pigment. However the calcinations are generally carried out by heating at least to 1300° C. for about 30 hrs.
Embodiments of compositions comprising materials satisfying the general formula AM1-xM1xM2yO3+y are disclosed, along with the methods of making the compositions in some cases the M and M1 cations in trigonal bipyramidal coordination, and the material is chromophoric. In some embodiments, the material is YIn1-xMnxO3, X is greater than 0.0 less than 0.75, and the material exhibits a surprisingly intense blue colour (U.S. Pat. No. 8,282,728 B2, Oct. 9, 2012).
Solid solutions of Co and Mg diphosphates with compositions Co2-xMgxP2O7 (x=0, 0.1, 0.2, 0.3, 0.5, 0.7, 1.0, 1.5 and 1.8) have been prepared and characterized by M. Llusar et al. (M. Llusar, A. Zielinska, M. A. Tena, J. A Bardenes, G. Monros Journal of European Ceramic Society 30 (2010) 1887-1896) for the first time as alternative low-toxicity blue ceramic pigments. The compositions were prepared through the conventional co-precipitation route and calcined up to 1000° C./2 h. These optimal compositions containing a minimized Co amount (measured values around 7-16 wt %) could be therefore less toxic alternatives to the conventional Co3(PO4)2 blue ceramic pigment are now under development. These pigments possess relatively less −b* value than CoAl2O4. The value of L*, a+, b+ and NIR solar reflectance (%) of commercial CoAl2O4 are 44.8, 2.1, −32.7 [M. Ocaila, J. P. Espinos, J. B. Carda, Dyes Pigm., 91, 2011, 501-507] and 29%[S. P. Radhika, K. J. Sreeram, B. U. Nair, J. Adv. Ceram., 1, 2012, 301-309].